<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>圆角边框</title>
    <style>
        #div1{
            width: 100px;
            height: 100px;
            border: 2px solid red;
            /*顺时针，左上开始*/
            border-radius: 50px;
        }
    </style>
</head>
<body>
    <div id="div1">

    </div>
</body>
</html>